Optical thin films are useful for a wide range of optical applications such as antireflective (“AR”) coatings, high-reflective (“HR”) coatings, dielectric mirrors, and thin film interference filters. Compact size and environmental stability are two properties of optical thin films that have stimulated their deployment in modern applications including optical communications, lighting, vision, instrumentation, medical devices, and display systems. Optical thin films typically manipulate light by interference, which is an additive or subtractive process in which the amplitudes of two or more overlapping light waves systematically attenuate or reinforce one another. This interference can provide polarization, wavelength-selective transmission and reflection, beam splitting, or various other effects on a light beam, according to the design of the thin film and its interaction with adjacent features in an environment of an optical system.
Harnessing the interference phenomenon with an optical thin film relies on precise control of the optical thin film's physical dimensions and material properties. A slight variation in the thickness of an optical thin film can significantly impact the thin film's optical performance. Similarly, anomalies in an optical thin film's material can cause unwanted variation or degradation in optical performance. This susceptibility to performance variation usually imposes tight tolerances on a process, such as a vacuum deposition process, that fabricates optical thin films.
Vacuum deposition processes can be difficult to control to a level that is sufficient to produce consistent optical thin films for applications that require high performance. For example, the yield for fabricating twenty five gigahertz (“GHz”) thin film band-pass filters for dense wavelength division multiplexing (“DWDM”) can be low. In one technique for controlling a thin film deposition process, an optical instrument monitors the deposition process by observing the buildup of thin film material in a region of a deposition chamber. Relative to the total surface area of optical thin film generated in a deposition batch, the monitored region may be relatively small. While the monitored region of thin film may provide acceptable optical performance, the unmonitored regions may exhibit performance characteristics that are out-of-specification in relation to their spatial separation from the monitored region. In certain circumstances, the acceptability rate of the output of a thin film optical filter batch can be less than twenty-five percent. Since conventional processes for producing optical thin films typically lack a provision for adjusting an optical property of an optical thin film in a controlled fashion after deposition, optical filters that do not meet acceptability standards of optical performance are frequently discarded as scrap.
Optical thin films that meet performance specifications and that are not discarded as scrap are often deployed in optical systems as discrete components, wherein the optical thin film adheres to a substrate or is freestanding, for example supported only on its edges. As a deposition substrate, a plate of glass or other optical material provides an optical thin film with structural support that facilitates handling and placement into the optical system. Integrating an optical thin film into an optical component that passively or actively manipulates light provides an alternative configuration that can deliver two or more optical manipulations in a single, integrated component. This integration can reduce the total size of an optical system, improve reliability, and streamline assembly. Optical components that are compatible with integrated thin film optical filters or other optical thin films include lens, lasers, optical amplifiers, gradient index lenses, optoelectronic components, optical fibers, detectors, displays, and planar light guide circuits (“PLCs”), for example.
While offering certain benefits, depositing thin films onto such optical components often imposes tight tolerances upon the fabrication process of the optical thin film. The optical characteristics of conventional thin films are typically defined upon thin film formation. Those characteristics or properties are typically not readily modified following thin film formation in a controlled fashion by conventional processes. Thus, depositing an out-of-tolerance optical thin film on an optical component can result in discarding or scrapping the integrated unit. Scrapping integrated optical assemblies is problematic and wasteful as optical components are often expensive, and convention deposition processes frequently provide a low yield of optical thin films having acceptable or desirable optical performance. For example, depositing a stack of optical thin films onto an end face of an optical fiber combines a filter function with a waveguide function into a single, integrated optical element for which defective performance is undesirable.
As an alternative to depositing thin film filters onto fiber end faces, Bragg gratings can provide integrated filtering in waveguides such as optical fibers and PLCs. Such a Bragg grating comprises an undulated refractive index disposed in the path of light propagating in the waveguide. That is, the waveguide exhibits a periodic fluctuation in the refractive index of its core and/or surrounding cladding. The modal field of light guided in the waveguide interacts with that core or cladding material
Illuminating a PLC or a fiber-optic waveguide with a pattern of ultraviolet (“UV”) light corresponding to a desired refractive index undulation “writes” the Bragg grating into the waveguide. The term ultraviolet or UV, as used herein, refers the region of the electromagnetic spectrum or light spectrum having a wavelength shorter than approximately 450 nanometers. Etching a corrugated, surface relief pattern into the waveguide is an alternative approach to forming a Bragg grating in a waveguide. Conventional Bragg gratings have been etched into the surface of glass PLCs that have ion-exchanged waveguides. An overcoat of optical material such as silicon dioxide or silicon oxynitride environmentally seals the exposed grating and the waveguide to prevent contamination that can compromise optical performance of the system. Conventional uses for this PLC system include grating-based stabilization of external cavity lasers and grating-based optical add drop multiplexing (“OADM”). The optical properties of the etched and the UV-written type of Bragg gratings, in PLCs or in optical fibers, can be changed conventionally through exposure of UV light and/or thermal energy.
While the optical properties of conventional Bragg gratings are changeable via conventional methods after forming the primary grating features, Bragg gratings exhibit attributes that can be undesirable in many applications. Relative to thin film optical filters, Bragg gratings are often expensive and susceptible to optical drift resulting from temperature changes and other environmental influences. In comparison to the physical thickness of a thin film filter, light typically propagates through a long physical distance of a Bragg grating to achieve an acceptable level of light manipulation. Also, in a band-pass configuration, thin film filters typically provide more desirable optical performance characteristics than Bragg gratings offer. Furthermore, whereas Bragg gratings are ordinarily embedded in select optical materials that are amenable to the grating-generation process, thin films can be applied to a wide variety of optical materials, substrates, and components, with minimal impact on the substrate.
In addition to passive optical components such as optical fibers, optical thin films have been applied to the facets of optoelectronic components such as semiconductor gain media devices, including semiconductor lasers and semiconductor optical amplifiers (“SOAs”). Applying a HR coat to one facet of a Fabry-Perot laser and an AR coat to the opposite facet and placing a wavelength-selective reflector, such as a fiber Bragg grating, in front of the AR-coated facet can establish an external cavity laser system that emits monochromatic light in the format of a single longitudinal mode. The proper function of this external cavity laser system typically requires the AR coat to provide a high level of performance. That is, the AR coat should minimize the reflection of light from the facet to a level that does not degrade the optical performance of the laser system. Achieving a suitable level of suppression of the facet reflection based on conventional technology can be challenging. Exacerbating the problem, the refractive indices of most semiconductor gain media are significantly higher than air, the typical media surrounding the facet.
A conventional approach to applying an AR coating to a laser, SOA, or other semiconductor gain media, entails depositing AR thin film layers onto a batch of Fabry-Perot laser dies or similar components, for example in a bar form, in a deposition chamber. An electrical supply in the chamber may deliver current during the deposition process to one of the laser dies in the batch. Instrumentation coupled to the laser monitors the laser's active or dynamic response to the application of the AR coat, thus inferring the optical performance of the AR coat. An operator can adjust deposition parameters in the chamber during the deposition process in an attempt to maximize AR performance, by controlling the refractive index of one or more layers and/or controlling the thickness of one or more layers. This conventional process typically suffers from several drawbacks that adversely impact yield and AR performance. Due to deposition variations associated with the spatial position of each laser die in the chamber, the AR coating on the monitored laser die may differ from the AR coatings on the other laser dies in the batch. An undesirably large number of optical thin film layers may be needed to achieve a specified level of AR performance to overcome lack of refractive index control in each layer. Also, coating a laser die with an out-of-spec AR coat can result in scrapping or discarding the laser die, which is wasteful or financially undesirable.
While maintaining deposition consistency in a coating chamber is desirable for applications such as applying AR coats to laser dies, physical thickness variation can also be purposely introduced to a thin film during the deposition process. The thickness variation can cause a corresponding variation in the manner in which light interacts with the coat. Although optical thin films that have uniform optical properties in a dimension parallel to the surface of the optical thin film are suited to many applications, other applications benefit from thin films with optical properties that vary in this dimension.
An optical thin film system that exhibits a spectral performance that varies in a dimension perpendicular to the thickness of the optical thin film offers utility for certain optical system applications. For example, a thin film optical filter can have a pass band that varies in a graded manner along the plane of the filter. Positioning an array of optical-fiber-coupled gradient index lenses adjacent to the filter plane filters the light associated with each lens according to each lens's spatial position on the filter.
A conventional process for forming such a filter with graded spectral properties entails varying the deposition rate across the optical filter's substrate during the deposition process. Grading the flux of particles across the substrate during the deposition process creates each thin film layer with a physical profile of varying thickness. That is, more deposited material yields greater physical thickness at various locations of each layer. A filter fabricated with this process has thin film layers of graded physical thickness. Since physical thickness of layers can correlate with the spectral position of a filter's pass band position, a graded physical thickness can yield a thin film optical filter that has different spectral properties at different physical locations on the filter.
This conventional approach to graded thin film filters has disadvantages for many applications. Since the spectral properties are defined in the deposition process, there is typically no provision for modifying those properties in a controlled manner after the deposition process is complete. Yield problems can result from inadequate control of the deposition process, for example. Also, the process typically produces thin films and thin film optical filters that have varying thickness, so the outer surface of such a thin film optical filter slopes relative to the substrate surface to which the stack of thin films adheres.
To address these representative deficiencies in the thin film art, what is needed is a capability for adjusting one or more optical properties of an optical thin film, such as an optical thin film in a thin film optical filter. Further a capability is needed to impart an optical thin film with a pattern, such as a refractive index or spectral pattern that varies in a graded fashion in a dimension parallel to the thin film's surface. Such capabilities would enhance the precision with which an optical thin film manipulates light and facilitate the cost-effective utilization of optical thin film technology in numerous applications.